1. Field of the Invention
The invention relates to imaging devices and, more particularly to imaging devices with enhanced dynamic range.
2. Description of the Related Art
Imaging systems, especially those in the visible and infrared bands, have many modern applications in various fields including personal digital photography, astronomy, medical imagery, surveillance, security and military target acquisition. Such systems are based on image sensors that convert light into an electrical signal. Light sensing visible imager sensors have become increasingly popular in recent years, especially in digital still cameras and video camcorders. Their popularity has been fueled by the development and availability of new cost-effective image sensor technologies.
Charge-coupled devices (CCD) and complimentary metal-oxide semiconductor (CMOS) are two widely used technologies for fabricating image sensors.
CCDs are an integrated circuit with an array of light-sensitive capacitors that are linked or coupled together. CCD signals require special off-chip processing, which increases the cost of a CCD imaging system.
CMOS image sensors are devices that use complimentary and symmetrical pairs of n-type and p-type field-effect transistors to perform basic logic functions. CMOS technology is used to create microprocessors, microcontrollers, static memory, data converters, amplifiers and other digital and analog circuits. One type of image sensor that can be made using normal CMOS processes is an active pixel sensor (APS). APS imagers include an array of pixels each of which comprise a photodiode to collect the input signal and multiple transistors to buffer and amplify the signal for output.
Although CCD devices currently offer a superior dynamic range, CMOS devices are more cost-effective because they use standard semiconductor processes and offer higher level of integration on a single chip. Due to advantages of CMOS image sensors, they are more desirable for consumer products. However, efforts to make the chips more cost-effective by making them smaller have dramatically shrunk pixel size leading to a much reduced dynamic range and a degradation of CMOS image sensor performance.
Dynamic range is one characteristic of an imager that indicates how well the device captures both the highlights and shadows of a scene. The dynamic range of a pixel is defined as the ratio of the saturation level (the highest useful signal) to the noise floor (the lowest observable signal). The dynamic range of an imaging system is sometimes referred to as the scene dynamic range. This refers to the range between the brightest level of illumination that can be detected and the darkest level of illumination that can be detected. The more sensitive a sensor is, the faster it will reach the saturation level. It is therefore desirable to design a system that can accommodate high sensitivity sensors without sacrificing information due to saturation.
There are several methods that attempt to realize a high sensitivity imager with a high dynamic range. One such method that minimizes information loss entails regulating the pixel integration time. U.S. Pat. No. 6,831,689 to Yadid-Pecht describes a theoretical approach to improving the dynamic range of an imaging system by controlling the integration time of each pixel in a sensor array. The method entails using a decision buffer to provide an indication as to whether the pixel is close to saturation. If the pixel is close to saturation, the pixel is reset and begins integrating again. The reset process may be repeated up to three times per pixel during a single integration period. A digital memory keeps track of all the resets that occurred for each individual pixel. The drawback of this theoretical solution is that the system would have to support and account for several resets per pixel during a single integration/read cycle. This requires several comparisons to be made, necessitating extra memory space and more complex circuitry to process and track the multiple resets. The increased memory and processing demands can decrease the system speed, increase its power and substantially increase cost.
The theoretical approach described above does not fully satisfy the need for a visible imager with an enhanced dynamic range that is, capable of retaining information from the input signal. Furthermore, the reference does not disclose how the theory might be realized.